Multiple delay line based on an awg and different sections of a dispersive optical medium

ABSTRACT

Multiple delay line based on AWG with a feedback configuration by means of sections of a dispersive optical medium, in which, due to the effect of the dispersion of the sections of the dispersive optical medium, the use of simultaneous multiple wavelengths (WDM) allows a large number of delays to be achieved, that can be optically varied by means of the individual or joint tuning of the multiple wavelengths. The application of the present invention is in any field in which it is necessary to obtain a large number of delays for example, such as occurs in the field of optical shaping of beams for antenna grouping. Other fields of application are analogue-digital optical converters, optical time division multiplexing (OTDM) or the optical systems based on code multiplexing (CDMA).

OBJECT OF THE INVENTION

The present invention consists of a delay line based on an AWG (ArrayedWaveguide Grating or diffraction mesh with grouped waveguides) anddifferent sections of a dispersive optical medium that allow a largenumber of delays, corresponding to a large number of optical carriers,to be simultaneously generated. The values of the multiple delays willdepend on the specific wavelengths, on the separation between these andon the dispersion of the optical medium. The invention providesessential novel features and notable advantages for cost reduction withrespect to structures known and used for similar purposes in the currentstate of the art.

The application of the present invention is in any field in which it isnecessary to obtain a large number of delays for example, such ashappens in the field of optical shaping of beams for antenna grouping,where the obtaining of different progressive delay subsets is basic fortheir operation. Other fields of application would be digital analogueoptical converters, optical time division multiplexing (OTDM) or opticalsystems based on code multiplexing (CDMA).

BACKGROUND TO INVENTION

Antenna grouping allows the formation of radiation patterns withfeatures that cannot be obtained using simple antennas. Specifically,they allow functionalities such as beam-steering, beam shaping ondynamic low-point introduction based on amplitude control and the delayof the supply of the different elements that comprise the cluster. Forthis object they have been widely used in a large number of areas withinthe telecommunications field. The most common functionality of anantenna cluster is the orientation of the beam in different directionsin space, which, in the case of a linear cluster with constant spacing,is achieved by means of the introduction of a progressive delay (delaysthat maintain a constant difference between adjacent elements of thecluster) between the different elements of the cluster.

Thanks to the proportional reaction that exists between the delayintroduced in an electromagnetic wave and the corresponding phasedifference for the frequency of this signal, it is usual to effect thecontrol on the phase shifts undergone by the signals corresponding toeach element of the grouping instead of on the delays. When the beamshaping must be done for different frequencies or for signals ofsignificant bandwidth this proportionality between delays and phaseshifts is no longer valid, giving rise to the well-known phenomenon ofbeam squint and obligating the use of delay lines.

Traditionally the control of antenna clusters has been carried out bymeans of radiofrequency (RF) base band processing. However, at highfrequencies (millimetric wavelengths) and high bandwidths base bandprocessing becomes unviable, and those proposed in RF present severaldisadvantages, among them, limited bandwidth, high losses and greatcomplexity of interconnection, as the number of elements in the clusterincreases. Furthermore, the RF solutions based on delay lines usuallyimply high volume, weight and complexity.

To solve the problems previously described in recent years differentproposals for optical shaping of clusters have appeared, theclassification field of this invention [1]-[4].

The beam optical shaping architectures among other advantages presentlow weight and size, immunity against electromagnetic interferences, andespecially they allow wide instantaneous bandwidth to easily obtainedand true delay operation (TTD, True Time Delay) which allows the beam tobe steered independently of the frequency of operation.

At first the delays of the different optical architectures were effectedby means of signal propagation through a given section of fibre [5].Later the use of the fibre dispersion was proposed to simultaneouslyimplement multiple delays [6]-[7]. The use of other dispersive means ofpropagation instead of different lengths of dispersive fibre has alsobeen proposed, as is the case of diffraction networks with variableperiodicity [8]. The limitations as regards bandwidth due to thedispersion inherent in delay lines based on dispersive elements, can beresolved with different optical modulations tolerant to dispersion[3]-[9].

U.S. Pat. No. 5,793,907 proposes a delay line based on an AWG. Theproposal is based on a symmetric feedback configuration in an AWG (knownas loop-back, that is to say, the feedback is made between an outputport of the AWG and its corresponding input port) as had been previouslyproposed for its use as ADM (Add-Drop Multiplexer) (10]-[11].

In the aforementioned patent, the delays are obtained by means ofsections of length such that the propagation of the signal retards theoptical signal the time required. To steer the main beam of theradiation pattern it is necessary to achieve a progressive delay betweenthe different elements of the cluster, which obligates a delay line tobe introduced for each element of the cluster, so that, each delay linemust increase the length of its sections of fibre to achieve a constantprogressive delay.

Therefore, this delay line presents a serious disadvantage; it isnecessary to replicate the structure for every element of the cluster,which can make the system unviable for large clusters in view of thehigh cost of the AWG.

Bliographical References Mentioned:

-   [1] W. Ng, A. Walston, G. Tangonan, J. Newberg, J. J. Lee and N.    Bernstein, “The First Demonstration of an optically Steered    Microwave Phased Array Antenna Using True-Time Delay” Journal    Lightwave Technology, vol. 9 pp. 1124-1131, September 1991.-   [2] R. D. Esman, M. Y. Frankel, J. L. Dexter, L. Goldberg, M. G.    Parent, “Fiber Optic Prism True Time delay Antenna Feed” IEEE    Photonics Technology Letters, vol. 5, no. 11, pp. 1347-1349,    November 1993.-   [3] H. Zmuda, E. H. Toughlian, “Photonic Aspects of Modem Radar” Ed.    Artech House, 1994.-   [4] I. Frigyes and A. J. Seeds, “Optically Generated True-Time Delay    in Phased-Array Antennas”, IEEE Transactions on Microwave Theory and    Techniques, vol. 43, no. 9, pp. 2378-2386, September 1995.-   [5] A. P. Goutzoulis, D. K. Davies, “Hardware compresive 2-D fiber    optic delay line architecture for time steering of phased array    antennas” Applied Optics, vol. 29, no. 36, pp. 5353-5359 December    1990.-   [6] R. Soref, “Optica1 Dispersion Technique for Time-Delay Beam    Steering”, Applied Optics, vol. 31, pp. 7395-7397, Dec. 10,1992.-   [7] D. T. K. Tong and M. C. Wu, “A Novel Multiwavelength Optically    Controlled Phased Array Antenna with a Programmable Dispersion    Matrix”, IEEE Photonics Technology Letters, vol. 8, no. 6, pp.    812-814, June, 1996.-   [8] J. L. Corral, J. Marti, S. Regidor, J. M. Fuster, R.    Laming, M. J. Cole, “Continuously Variable True Time Delay Optical    Feeder For Phased Array Antenna Employing Chirped Fiber Gratings”    IEEE Transactions on Microwave Theory and Techniques, vol. 45, no.    8, pp. 1531-1536, 1997.-   [9] G. H. Smith, D Novak, Z. Ahmed “Novel technique for generation    of optical SSB with carrier using a single MZM to overcome fiber    chromatic dispersion”, Intenational Topical Meeting on Microwave    Photonics, Kyoto (Japan) Paper PDP-2, December 1996.-   [10] Y. Tachikawa Y. Inoue, M. Kawachi, H. Takahashi and K. Inoue,    “Arrayed-Waveguide Grating Add-drop Multiplexer with Loop-back    Optical Paths”, Electronics Letters vol. 29, no. 24, pp. 2133-2134,    November 1993.-   [11] Y. Tachikawa and M. Kawachi, “Lightwave Transrouter based on    Arrayed-Waveguide Grating Multiplexer”, Electronics Letters, vol.    30, no. 18, pp. 1504-1505, 1^(st). Sep. 1994.-   [12] M. K. Smit, “New Focusing and Dispersive planar component based    on an Optical Phased Array” Electronics Letters, vol. 24, no. 7, pp.    385-386, March 1988.-   [13] C. Dragone, “An N×N Optical Multiplexer Using a Planar    Arrangement of Two Star Couplers”, IEEE Photonics Technology    Letters, vol. 3, no. 9, pp. 812-815, September 1991.-   [14] C. Dragone, C. A. Edwards and R. C. Kistler, “Integrated Optics    N×N Multiplexer on Silicon”, IEEE Photonics Technology Letters, vol.    3, no. 10, pp. 896-899, October 1991.

SUMMARY OF THE INVENTION

Specifically, the present invention combines the properties ofcommutation according to the wavelength of the AWG together with thewavelength periodicity of its performance and the capacity of an opticaldispersive medium to differently retard different wavelengths.

More particularly, the invention proposes the use of an AWG in afeedback configuration (each optical signal propagates twice through theAWG) with sections of a dispersive optical medium, together with asource that could switch between different wavelength subsets (WDM,Wavelength Division Multiplex) and it might even combine differentwavelengths of every subset. Within each subset the differentwavelengths act so that the separation between them is equal to thespectral periodicity of the AWG (FSR, Free Spectral Range) so that,after all the wavelengths of the subset are introduced through a port ofthe AWG, all of them are routed towards the same output port. In thebasic configuration of the delay line the selected output port connectsby means of a given length of dispersive optical medium with a specificone of the inputs or outputs (that selected or a different one) of theAWG, so that the dispersive medium differently retards each of thewavelengths of the subset before passing again through the AWG and berouted towards the common output port. The relative delays between thedifferent wavelengths will depend on the spectral separation betweenthese and on the total dispersion of the dispersive medium. If each ofthe outputs of the AWG connects to a dispersive medium with differentdispersion parameters, the selection of the suitable wavelengths subsetwill allow the values of the different delays associated with eachoptical carrier to be selected.

In addition, the multiple delay line subject of this invention obtainsmultip1es delays by means of the routing of different optical carriersthrough the same section of dispersive optical medium, benefiting fromthe periodicity of the transmission response of the AWG. In theloop-back configuration, if through one input of the AWG different FSRseparated optical carriers are introduced, all of them are routedtowards the same output and therefore they will feedback to the AWGthrough the same input, range over, therefore, the same section. If thissection corresponds to a dispersive medium such as might be a givenlength L(m) of fibre with a constant dispersion D (ps/nm·m), two opticalcarriers that were separated Δτ(nm) would undergo a relative delaybetween both of value:Δτ(ps)=D(ps/nm·m) L(m)·Δλ(nm)

In this way, if as many wavelengths as the cluster has elements areintroduced into the delay line, with a separation between them constantand equal to the FSR of the AWG, it is possible to introduce therequired delay between carriers by means of adjustment of the totaldispersion of each section of dispersive optical media that form thefeedback, which, for the case of optical fibre with constant dispersion,is equivalent to adjusting the length of each section. At the output ofthe delay line, the different wavelengths will de-multiplex, so thateach wavelength will feed to one element of the cluster. On establishingthis one-to-one correspondence between wavelengths and elements of thecluster it is possible, in principle, to use a single AWG for the wholecluster, independently of its number of elements. In practice alimitation appears, due to the number of orders (number of channels in aFSR) available from the AWG.

Although the most usual embodiment of this invention would be the use ofa subset of wavelengths FSR separated from each other, it would bepossible to introduce the wavelengths with a separation that was notequal to the FSR of the AWG but rather at arbitrary multiples of this,so that multiple arbitrary delays of the simple delay are achieved,understanding by simple delay that which corresponds with that due tothe dispersive effect between two wavelengths separated the FSR of theAWG. The simultaneous selection of one or several carriers of more thanone of the subsets would even be possible, allowing a greaterflexibility in the selection of the delays at the cost of greatercomplexity in the optical generation and de-multiplexing stages.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics and advantages of the invention will bemade more clearly evident from the following detailed description of apreferred form of embodiment, provided solely as an illustrative andnon-limitative example, with reference to the accompanying drawings, inwhich:

FIG. 1 shows the schematic layout of an integrated AWG, basic device ofthe delay line set forth herein.

FIG. 2 is a detail of the free propagation region of that shown in theprevious figure.

FIG. 3 shows the schematic layout of the delay line. It relates to anAWG in what it is known as loop-back configuration. In contrast toprevious proposals, the feedback is carried out with a dispersiveoptical medium.

FIG. 4 shows the spectrums of the optical signals at the input of thedelay line.

FIG. 5 shows an example of full architecture of a cluster of antennasbased on an optical shaper that includes the multiple delay line basedon AWG with dispersive fibre sections.

FIG. 6 shows the schematic layout of the delay line in a fold-backconfiguration (feedbacks between AWG output ports) appropriate when adispersive medium operating in reflection mode is used.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to perform the detailed description of the preferred embodimentof the present invention that follows, continuous reference will be madeto the drawings of the Figures, throughout which the same numericalreferences for equal or similar parts have been used. Thus, FIG. 1refers to the principal component of the architecture, the AWG, formedby free propagation regions (2 and 4) joined by a cluster of waveguidesof different lengths (3) and a set of access waveguides at each of theends of the free propagation regions (1 and 5). FIG. 2 is a detail ofthe free propagation region (2 and 4) of the AWG. In this, Δ representsthe angle of divergence between the input and output waveguides, R_(a)is the focal length, d_(a) is the separation between waveguides of thecluster of guides (6), d_(r) is the separation between waveguides of theoutput guides (5) and θ represents the resulting angle of dispersion ofthe phase difference between adjacent guides.

In addition, FIG. 3 shows the multiple delay line, subject of thisinvention, in a loop-back configuration formed by an AWG (12) and asmany sections of dispersive optical media (8, 9, 10 and 11) as thenumber AWG ports less one, a port that corresponds to the common inputand output ports.

In FIG. 5 the full architecture is shown, comprising of: the multipledelay line of FIG. 3; the optical source (7), that must provide multiplewavelengths, as is shown in FIG. 4; a data source (15) and anelectro-optic modulator (14), a de-multiplexer (22) that separates thedifferent wavelengths and assigns them to the correspondingphotoreceptor (23, 25, 30 or 34) to the appropiate element of thecluster (27, 28, 31 or 33). Since it is a reciprocal device this delayline can be used both in the transmission mode and in the reception modeof the antenna cluster, it being only necessary to add a series ofseparation and combination devices together with the correspondingoptical-electrical and electrical-optical conversion stages;specifically it will be necessary to add: a de-multiplexer (35), asplitter (16) that sends the multiple carriers in the route (18) fortransmission mode and in the route (17) for the reception mode acombiner (37), a pair of diplexers (19 and 20), a number of electricalcirculators (24, 26, 29, 32) a number of electro-optic modulators (36,38, 39, 40) and a photoreceptor (21) as is shown in FIG. 5.

FIG. 6 shows the multiple delay line, subject of this invention, in afold-back configuration formed by an AWG and as many sections (41) ofdispersive optical media, actuating in reflection mode, as the number ofAWG ports.

The basic element of the multiple delay line proposed in this inventionis the AWG schematically shown in FIG. 1.

The operation of said device has been widely dealt with in theliterature [12]-[14] being the following: the signal enters through oneof the optical input guides (1) henceforward input ports. When thesignal reaches the free propagation region (FPR Free Propagation Region)(2) it is no longer laterally confined and diverges. When it reaches theinput opening the beam links to the waveguides cluster (3) and itpropagates through the individual waveguides up to the output opening(5) in the second free propagation region (4).

The length of these guides is chosen so that the difference in lengthbetween adjacent guides is equal to a whole multiple of the centralwavelength of the ANG. For this wavelength, the fields in the individualwaveguides (3) will reach the output with the same phase, apart from awhole multiple of 2π, and the field distribution that we had in theinput opening will be reproduced in the output one. Therefore thedivergent beam in the input opening is transformed in a convergent one,with equal amplitude and phase distribution to that of the output. As aconsequence of the dispersion introduced by the grouping of waveguides(3) the output beam will incline and the focal point will move along theimage plane.

According to that shown in FIG. 2, by placing receiving waveguides (5)in suitable positions along the image plane we achieve spatialseparation between the different wavelengths. That is, if the wavelengthchanges, the focal field of the AWG moves along the receivingwaveguides. The frequency response of the different channels of thesuperposition of this field with the modal fields of the receivingguides being obtained. The periodic behaviour of the AWG is translatedinto that two FSR separated wavelengths imply a displacement of thefocal field towards the same receiving waveguide, routing bothwavelengths towards the same output.

In accordance with the invention and in the light of FIG. 3, by means ofthe source (7) a set of wavelengths will be chosen, that will beintroduced through the common input port of the delay line (8). Withinthe set there will be as many wavelengths as delays it is wished togenerate, and the separation between them will correspond with multiplesof the FSR of the AWG, so that all of them are routed towards the sameoutput port. Therefore, all the wavelengths will go through the samesection of dispersive medium. The choice of a section (with its fullassociated dispersion) and consequently of the large number of delays,is carried out optically by means of the choice of a specific subset ofwavelengths. In the case that the dispersive medium is optical fibrewith a constant dispersion, the expression (1) shows that a given delaywill be introduced between the different wavelengths, due to thedispersion of the fibre. Although the dispersive medium will preferablyhave a linear delay response to the wavelength, any other response (forexample such as: curved, parabolic or sawtooth) is possible and viablewith the current state of the technology.

Because of the feedback configuration, the set of wavelengths will againenter one of the input ports of the AWG (1). Due to the symmetricalbehaviour of the AWG, this will route all the wavelengths towards thecommon output port (13).

At the output of the multiple delay line a de-multiplexer (22) isintroduced, that separates each wavelength directing it towards aphotoreceptor (23, 25, 30 or 34). The output of each photoreceptor willsupply an element of the cluster (27, 28, 31 or 33). In this way, themultiple delay line has introduced, with a single AWG, a progressivedelay between elements of the cluster, which allows its steeringdirection to be changed.

In case of using dispersive mediums that operate in reflection mode, asis the case of diffraction networks with variable periodicity (CFG,chirped fiber grating) the configuration of FIG. 3 would be slightlymodified, becoming as is shown in FIG. 6, its operation being completelyequivalent to that of FIG. 3.

As regards the optical source (7), this must be capable of providing aspectrum similar to that shown one in FIG. 4. Different ways ofgenerating this type of spectrum exist, such as, for example, by meansof commutation between different multi-wavelength lasers with anadequate separation between carriers. Although the preferredimplementation implies the selection of subsets of optical carriers witha spectral separation of whole multiples of the FSR, the simultaneousselection of one or several carriers of more than one of the subsets ofFIG. 4 is possible, allowing a greater flexibility in the selection ofthe delays at the cost of greater complexity in the optical generationand de-multiplexing stages.

It is not considered necessary to make the content of this descriptionmore extensive in order that an expert in the matter may understand itsscope and the advantages derived from the invention, likewise to developand to put into practice its object.

Nevertheless, it must be understood that the invention has beendescribed in accordance with a preferred embodiment of it, because ofwhich it can be liable to modification without this implying anyalteration in its basis, set out in the attached claims.

1. Multiple delay line based on AWG and different sections of andispersive optical medium, characterized in that by means of the use ofsimultaneous separated multiple wavelengths (WDM) the FSR of the AWG iscapable of introducing a different delay for each optical carrier, thedelays being obtained due to the dispersion of the sections of thedispersive optical medium of the feedbacks, and because the obtaining ofone or another set of delays is optically performed by means of thechoice of a specific subset of wavelengths.
 2. Multiple delay line basedon AWG according to claim 1, characterized in that the dispersiveoptical medium consists of sections of dispersive optical fibre,diffraction networks or any other dispersive medium both in transmissionand in reflection.
 3. Multiple delay line based on AWG according toclaim 1, characterized in that the feedback configuration between theinput and output ports of the AWG is any type of loop-backconfiguration, both those that connect a output port with itssymmetrical input one, and those that connect the input and output portsin any other way.
 4. Multiple delay line based on AWG according to claim1; characterized in that the feedback configuration between the inputand output ports of the AWG is any type of fold-back configuration, inwhich the output ports of the AWG connect with each other and even withthemselves.
 5. Multiple delay line based on AWG according to claim 1,characterized in that the wavelengths have a separation between themthat is not the FSR of the AWG but rather multiples of this.
 6. Multipledelay line based on AWG according to claim 1, characterized in that afinite number of wavelengths groups are simultaneously introduced, sothat the wavelengths of each group are FSR separated from each other,but not in this way from the wavelengths of other groups.